"There's an OK amount of footage where you can actually see their performances," Stewart said of learning guitarist Joan Jett's moves, speaking at a Sundance round table in support of the film.

The actress also calls jamming with an electric guitar "exhausting."

"I would finish rehearsal and be like, 'My shoulders are killing me!' "

Fanning got off a little easier, because her character, Cherie Currie, was lead singer. There's a powerful scene, however, in which Fanning does some signature Cherie microphone tricks on stage. (Check out Currie demonstrating the real thing in the video below, around 1:07.)

"Cherie taught me how to do that, which was really a special moment," Fanning said.

Director Floria Sigismondi had Fanning the longest in prep time, where she'd bring the young star to rehearse with her husband's band Living Things. Stewart and the supporting young women in the band were given guitar and drum lessons.

When production ramped up, everyone was brought together over two weeks to rehearse and record the tracks covered in the movie.

"You can never really feel what it's going to be like until you actually film it," Fanning said, referring to the loud and large type of shows the Runaways lived through in the '70s.

From what's out there, do you think the girls look convincing as musicians? Tell me in comments!